1. Field of the Invention
This invention relates to dredgers for removing sand, silt and like material from the river or sea bed and has application, for example to clearing wrecks, and providing trenches in which, for example, pipelines may be laid.
2. Description of the Related Art
A suction dredger is the most widely used apparatus for removing such material, suction being created by a motor and pump unit, somewhat like a vacuum cleaner. However, if used for clearing wrecks, such apparatus has the disadvantage that small and/or lightweight articles from the wreck can also be lifted and, even if a screen is provided in the suction path, the articles may be small enough to pass through the screen, or be difficult to extract from the other debris lifted.
A modified form of the suction method, which is used in tidal waters, is to provide a vertical length of pipe above the area to be cleared near the lower end of which air is fed under pressure to pass upwardly through said pipe. This creates a vacuum, which will act to lift the sand, silt and like material and set it in suspension with the water, whereafter it may be carried away from the area by the tide. This method is reliable in reducing the possibility of small/lightweight articles being lost, but is time consuming due to the relatively small diameter of the pipe, normally around 0.5 metres, and hence restricted area covered.
Another method, which can be used in relatively shallow tidal waters, e.g. up to about 10 metres in depth, comprises mooring a tug, ship or other vessel in a fixed position above the area to be cleared and deflecting the propeller wash downwardly using a suitable guide plate. The wash disturbs the material around the wreck, which material is thereby lifted, set in suspension and carried away from the area by the tide. Apart from the shallow depth, another restriction of this method is that, for a large wreck, the position of the vessel must be changed progressively to cover the complete area of the wreck, which is difficult and time consuming.
EP-A-328 198 disclose a method of dredging in flowing water comprising lowering a casing of a wing shape downwardly towards the area to be cleared, the casing carrying thrust means arranged so that the thrust means is directed downwardly, the orientation of the wing casing being adjusted in the water so that it presents a surface relative to the flow which causes a resultant downward vertical component of force to counteract the upward force provided by the thrust means, the thrust means directing a wash of water towards the areas to be cleared so that the turbulence created clears the sand, silt or like material covering the area.
This method of dredging is particularly useful for providing a trench across the sea bed. The wing shape casing is slowly towed along a line above the sea bed and the thrust means, which is directed vertically downwards, excavates a trench in the sea bed of a width which depends upon the material of the sea bed, its altitude above the sea bed, the power in the thrusters, its speed over the sea bed, and its pitch angle. In a typical example, the width of trench formed will be of the same order as the width of the wing shape casing.
Such a dredger, which is commonly known as a "wing dredger" has been successful in producing a trench of a width sufficient to take a pipeline or, alternatively, to flatten an area of sea bed in preparation for works on the sea bed. However, its usefulness could be increased if it could, on occasions, provide a trench or clear an area of the sea bed of a greater width. This would allow a single wing dredger to be used in a wider variety of circumstances or alternatively a smaller wing dredger to be used to provide a trench of a particular width.
One's initial view might be that increasing the thrust with accompanying changes in pitch angle, or even the addition of ballast weights to the wing would provide a wider trench but in practice this would simply provide a deeper trench. Also an increase in thrust may mean that the wing has to be larger so as to counter-balance the upward reaction of the downward thrust.
A second proposal would be to increase the size of the wing dredger and provide more thrust means. This would provide a more unwieldy and more expensive wing dredger. This is undesirable for the limited additional use that such a wing dredger would have for providing trenches of greater than normal width, and also because the wing dredger has to be transported, typically over land, between assignments. A known wing dredger already requires three lorries to move the various sections, and, for example, doubling in size would require six lorries to carry out the same transportation with attendant increased cost.